Direct Synthesis of Metal-Organic Frameworks on Microresonators and Their Gas Sensing Application

Abstract

In this thesis, the direct synthesis routes of various kinds of metal-organic frameworks (MOFs) are introduced. Addressing the drawbacks associated with the conventional MOF synthetic routes, various metal-oxides used as a metal-ion supplier were converted to the MOFs structures. In addition, the MOFs structures were integrated with the microcantilever and quartz crystal microbalance (QCM) for investigating their vapor sorption characteristics. This thesis consists of 4 chapters. Chapter 1 introduces the general background about MOFs synthesis, microcantilever, and QCM which the most part of the thesis is focused on. Chapter 2 through Chapter 4 are research works about the development of various MOFs structure on microcantilever and QCM for diverse applications. Chapter 2 introduces nanoporous ZIF-8 microcantilever gas sensor derived from zinc-oxide nanorods. The zinc-oxide nanorods were synthesized and converted to nanoporous ZIF-8 films on silicon microcantilever. The dense, uniform and intergrown ZIF-8 films were successfully synthesized on only one side of microcantilever without the assistant of chemically functionalized surfaces. The changes in resonance frequency and deflection are measured simultaneously. During the exposure of methanol, ethanol, or 1-propanol, the sorption kinetics in resonance frequency and deflection were different. The changes in frequency indicated the diffusion of alcohol molecules in the framework and the changes in deflection indicated the binding of alcohol molecules to hydrophobic organic linker in the framework. The diffusion occurred very fast and the binding occurred slowly. The adsorption of alcohols were governed by the binding of molecules in frameworks. In addition, the sorption behavior was investigated by plotting resonance frequency with respect to deflection changes. This ZIF-8 microcantilever system elucidates the sorption behavior and the structure changes of ZIF-8 frameworks during the exposure of methanol, ethanol, or 1-propanol. In chapter 3, CO2-selective nanoporous metal-organic framework micro- cantilevers were developed. Nanoporous anodic aluminum oxide (AAO) micro- cantilevers were fabricated and MIL-53 metal-organic framework layers were directly synthesized on each cantilever surface by using the aluminum oxide as the metal ion source. Exposure of the MIL53-AAO cantilevers to various concentrations of CO2, N2, CO, and Ar induced changes in their deflections and resonance frequencies. The results of the resonance frequency measurements for the different adsorbed gas molecules were almost identical when the frequency changes were normalized by the molecular weights of the gases. In contrast, the deflection measurements show that only CO2 adsorption induced substantial bending of the MIL53-AAO cantilevers. This selective deflection of the cantilevers is attributed to the strong interactions between CO2 and the hydroxyl groups in MIL-53, which induced structural changes in the MIL-53 layers. Simultaneous measurements of the resonance frequency and the deflection were performed to show that the diffusion of CO2 into the nanoporous MIL-53 layers occurred very rapidly, whereas the binding of CO2 to hydroxyl groups occurs relatively slowly, which indicates that the adsorption of CO2 onto the MIL-53 layers and the desorption of CO2 from the MIL-53 layers are reaction limited. Chapter 4 covers direct synthesis of CuBDC on quartz resonators for enhanced n-hexane sorption under humid conditions. CuBDC was synthesized by facile and simple synthetic route on the metal copper layers. The sheet shape of CuBDC was successfully synthesized and perpendicularly formed on the metal copper layers. To investigate the reaction mechanism, the XRD and SEM analysis were performed by changing the concentrations of organic linkers. It was found that the preferential orientation of CuBDC on the substrates was caused by the diffusion-limited reaction by the dissolution of metal copper to produce the copper ions. The thermal behavior of CuBDC confirmed that the synthetic route is feasible on the metal copper layers. The n- hexane vapor sorption was conducted on the quartz resonators with varied humid conditions. The n-hexane was preferentially bound to the hydrophobic benzene groups in CuBDC frameworks and the water could not bind to the hydrophilic groups in CuBDC frameworks. The octadecyltrichlorosilane (ODTS)-treated CuBDC enhanced the sensitivity of n-hexane to 3 times.